Embodiments of the present invention concern a capacitively coupled plasma source for processing a workpiece such as a semiconductor wafer. A capacitively coupled plasma source comprises a ceiling electrode that is driven at a very high frequency (VHF) frequency over 110 MHz which can produce a high density plasma at a relatively low voltage. A capacitively coupled plasma source can further produce a low electrode potential for low electrode erosion, and permits the ion energy at the wafer surface to be limited to a low level if desired, while operating over a wide range of plasma density (very low to very high plasma ion density). One problem inherent in such a plasma source is that the ceiling electrode exhibits radial transmission line effects and loading due to the effective dielectric constant of the plasma. For example, at 150 MHz, a free-space quarter wavelength is about 20 inches, which is on the order of the diameter of the ceiling electrode (about 15 inches). Therefore, the RF field varies significantly across the surface of the ceiling electrode, giving rise to process non-uniformities at the wafer surface. For a plasma with an effective dielectric constant greater than 1, the effective wavelength is reduced to less than the ceiling electrode diameter, worsening the non-uniformity of the RF field, making processing non-uniformities across the wafer surface worse. For an etch process, this may produce a non-uniform edge low etch rate distribution across the wafer surface.
Various approaches are employed to reduce such undesirable effects. In one approach, magnetic steering may be employed to alter the plasma ion distribution, e.g., to reduce its center-high non-uniformity to produce a somewhat flatter distribution. One problem with this approach is that a center-high non-uniformity of the source may be beyond the corrective capability of magnetic steering. Another problem with this approach can be electrical charging damage of the workpiece if the magnetic flux density is too high. In another approach, the plasma sheath (or bias) voltage is increased by applying more plasma RF bias power to the wafer. This has the effect of increasing the plasma sheath thickness which in turn typically decreases the capacitance across the ceiling-plasma sheath as well as the capacitance across the wafer-plasma sheath, thereby forming three capacitors in series, including the ceiling sheath capacitance, the plasma capacitance and the wafer sheath capacitance. The net effect is to reduce the effect of the dielectric constant of the plasma, thereby reducing the non-uniformity of the RF field. The high bias voltage required in some oxide etch plasma process recipes is compatible with this latter approach. However, a high plasma bias voltage is not desirable in some other types of plasma processes. The worst non-uniformities appear in processes employing the lowest plasma bias voltage.
Such approaches are complicated by the fact that other process conditions dictated by the process recipe have as great an effect upon plasma distribution as either magnetic steering or bias (sheath) voltage. For example, increasing chamber pressure produces a less center high and a more center low plasma ion distribution, while decreasing the chamber pressure produces a more center high distribution. Other changes in plasma distribution are caused by source power (plasma density), gas chemistry, electronegativity of the gas mixture, pumping rate, gas flow rate and other parameters dictated by the process recipe.